1. Field of the Invention
The invention relates to the field of processes for measuring coating properties and, in particular, to a process for measuring the resistivity of a resistive layer of material hidden under layers of dielectric materials.
2. Description of Related Art
RAM coatings contain magnetic particles incorporated into a binder, such as urethane. The thickness and loading of the coating must be controlled in order to obtain the proper radar absorption properties. Also of importance is the ability to measure the resistivity of a resistive layer under a dielectric cover or tile. One approach is to use a hand held thickness measuring device as disclosed in U.S. Pat. No. 5,012,248 “Radar Absorption Material Thickness Measuring Device” by J. R. Munroe, et al. This invention comprises of a radiating element assembly for transmitting RF energy to and recovering reflected RF energy from the coating. A visual display is provided to indicate the thickness of the coating. A portable power supply is coupled to the detector assembly making it portable. This device is highly suitable for use in checking repairs made in the field.
It is common to apply RAM coatings by use of hand and robotic spray techniques. Since coating thickness is critical, it is desirable to check the coating thickness prior to it curing. This would make the by J. R. Munroe, et al. device unusable because of the damage to the coating that would occur upon contact of the device onto the wet surface. This problem can be avoided by the use of radiating and receiving electromagnetic transmission horns angled toward each other. The signal from the radiating horn is directed at the surface and the return signal is received by the receiving horn. Thus the measurement is limited to relatively large areas. This prevents accurate readings of significantly curved surfaces. Furthermore, it cannot be used in confined areas such as the engine inlet ducts on aircraft.
Conventional inspection techniques, such as those which use ultrasonic techniques, are unsuitable, for radar absorption is not measured, and because ultrasound does not propagate well in loaded urethane or silicon based materials. Thus it is possible that the measured thickness of the coating may be correct, but the area may not be properly loaded with magnetic materials.
In U.S. Pat. No. 6,788,244B1 Inspection Device For Radar Absorbing Materials by K. K. Tam, an inspection device is disclosed for non-contact inspection of RAM surface coatings containing radar-absorbing materials on a conductive surface or substrate. In detail, the device includes a first circuit for transmitting an electromagnetic signal to the assembly. The first circuit includes a radio frequency (RF) source of electromagnetic radiation coupled to a waveguide made of a conductive material coupled in series to a second waveguide made of a dielectric material with their longitudinal axes aligned. A second circuit is provided for receiving the portion of the electromagnetic radiation transmitted by the first circuit reflected from the assembly. The second circuit includes a third waveguide made of a conductive material coupled in series to a fourth waveguide made of a dielectric material with their longitudinal axes aligned. The second circuit further includes a RF power detector coupled to the third waveguide. Thus an electromagnetic signal is transmitted from the first waveguide to the second waveguide on to the assembly and the portion of the electromagnetic signal reflected off the assembly is received by said fourth waveguide and transmitted to said third waveguide and to the RF power detector. The longitudinal axes of the first and second waveguides are at an acute angle to the longitudinal axis of the third and fourth waveguides. This angle is preferably 10 degrees. However, this device operates on reflective magnitude only. Selective analysis of resistive layers is not possible.
In U.S. Pat. No. 5,355,083 Non Contact Sensor And Method Using Inductance And Laser Distance Measurements For Measuring The Thickness Of A Layer Of Material Over Laying A Substrate by A. R. George, et al. discloses the use of a laser to aid in the positioning of an Inductance type coating thickness measuring device. A pair of lasers is used to measure the thickness of a coating. One laser is designed such that beam passes through the coating onto the substrate wherein it is reflected back to a sensor. The second laser is designed so that its laser beam is reflected off the coating to a second sensor. A computer is used to process the two signals and thus determine the thickness of the coating. Such a system will only work when the coating is transparent to the first laser beam. It also depends upon having the laser beams in a fixed position. It would also be ineffective in a hand held device, since the two lasers will not provide sufficient feedback to obtain the proper distance from and orientation to the test surface.
Thus, it is a primary object of the invention to provide a process for determining the resistivity of a layer of resistive material.
It is another primary object of the invention to provide a process for determining the resistively of a layer of resistive material covered by a second layer of non-conductive material.
It is a further object of the invention to provide a process for determining the resistively of a layer of resistive material covered by a second layer of non- conductive material by use of time domain signal processing.